dinput: Implement DIPROP_KEYNAME property for keyboard device

[Sergey Khodych]

> In ascii function you are calling unicode function and copying it's output 
> (unicode string) to the ascii string buffer. This is wrong. Also because 
> these functions deal with strings you have to implement both ascii and 
> unicode functions at the same time. You can't forward one to the other.
Variable ps->wsz not a ascii string buffer because wsz field of struct
DIPROPSTRING has type WCHAR[MAX_PATH]. Function
IDirectInputDevice8::GetProperty works only with unicode strings.